The following Patent Document 1 discloses an airless tire, wherein a tread ring and a hub which are formed in advance are united with spokes during injection molding the spokes with a polymer material.
The following Patent Document 2 discloses an apparatus for measuring a stiffness of such a tread ring for airless tires, wherein a part in the circumferential direction of the tread ring corresponding to the ground contacting patch of the tire is pressed against a flat plate so as to be deformed, while the remaining part of the tread ring is supported not to be deformed, and
the stiffness in the tire radial direction of the circumferential part is measured over the entire circumference by rotating the tread ring in order to check uniformity of the tread ring in the circumferential direction.
On the other hand, when such an airless tire is mounted on a vehicle, a load of the vehicle is applied to not only the lower part of the tread ring but also the upper part of the tread ring through the spokes, namely, the upper part is pulled down by the spokes.
Therefore, when assessing the tread ring for such airless tire, it is preferable to determine the diametrical stiffness of the tread ring alone rather the stiffness of a circumferential part. If the diametrical stiffness is low and the amount of deflection of the tire becomes too large, there is a possibility that the spokes are damaged. If the diametrical stiffness is high and the amount of the deflection is too small, the ride comfort is deteriorated.
Thus, in addition to the uniformity in the circumferential direction of the tread ring, the diametrical stiffness is an important factor for the tire performances.
Therefore, it is necessary to measure the diametrical stiffness of the tread ring alone so as to make the airless tire to be appropriately deflected.